In order to melt fine metallic and/or metal-containing particles, in particular chips produced by machining, according to the prior art two types of induction furnaces are known. Both types of furnaces are based on the use of magnetic induction.
It is standard to melt metal chips, in particular brass chips, in an induction crucible furnace comprised of a heat-resistant crucible surrounded by a water-cooled copper coil. This coil is energized with alternating current so as to produce an alternating magnetic field in the crucible charge which causes it to melt. The thus produced alternating field creates an intense mixing of the melt which pulls in metal particles added from above. In this manner, since the often oil-covered metal chips are rapidly pulled into the melt, metal losses of all types are minimized and there is minimal generation of toxic carbon compounds.
The currents in the magnetic coil and in the melt produce together with the magnetic field forces directed along the axis of the cylinder so that the upper surface of the melt is convex. Slag deposits itself annularly around the upper melt surface on the inner wall of the furnace, the thickness of the slag ring being smaller with greater movements of the melt.
As a result of the process the described crucible furnaces have the following disadvantages:
First the thermal efficiency of the crucible furnace is relatively low so that specific energy consumption is high. In addition the crucible furnace can only work in batches. Once the crucible furnace is full, the melt must be poured off before more metal can be melted. This produces down times that substantially reduce the capacity of the unit.
As a result of deposits on the walls of the crucible there is substantial cleaning work. Finally slag deposits on the crucible wall lead to intolerable losses in efficiency.
An alternative is the so-called channel furnace where the melt is held in a closed channel around the iron core of a low-frequency transformer. The melt forms the short-circuited secondary winding so that heat is produced by the high currents flowing in the melt. Such a channel-type furnace does mix the melt so that there is the danger of scorching of the metal when metal particles lying atop the melt are exposed to an oxidizing atmosphere. Plungers or mixers can be used to reduce scorching of the metal, but this entails a technical expense. Although the thermal efficiency of a channel furnace is considerable, only small melts can be processed since the mechanical mixing takes quite some time. As a rule only about 30% of the melt can be metallic scrap chips in order to achieve acceptable efficiency. Even so, like crucible furnaces, the channel furnaces work discontinuously. This also has the disadvantage of considerable down time.